1. Field of the Invention
This invention relates to a monitoring circuit for constant monitoring of several signal inputs, at least two, particularly four.
2. Discussion of Background
This type of monitoring circuit is known from the state of the art. For example, they are used for monitoring reflective light barriers and polarisation filters. For this purpose, at least two sensors are tested cyclically at a frequency of for example 300 Hz. If during this test procedure, an obstacle is detected in one of the two beam paths, or an interference at one of the two redundant chains of the electronics or at one of the two sensors, two safety relays drop, which switch off a dangerous machine connected thereto. The whole system can be monitored via an external test input, the monitoring also including the relays. After a disruption of the protective field, i.e. after the machine has been switched off, switching on again is only possible through a manual release. A disadvantage of the above described monitoring circuit is that the relays used operate relative slowly due to their mechanical components, so that only specific monitoring frequencies can be reached. Moreover, the relays are basically subject to wear, i.e. burn out or the like can cause circuit modifications.